Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac-susceptibility, specific heat and muon-spin relaxation (µSR). Short-range static magnetism occurs below the freezing temperature Tg ≈ 0.7 K, which prevents the system from accessing the putative ferromagnetic quantum critical point. In the µSR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.PACS numbers: 71.27.+a, 64.70.Tg, 76.75.+i, 75.50.Lk A long-standing question in the field of quantum criticality is whether a ferromagnetic (FM) quantum critical point (QCP) generally exists and, if not, which are the possible ground states of matter that replace it. Quantum critical points occur when a material is continuously tuned with an external parameter (pressure, magnetic field, etc.) between competing ground states at zero temperature [1, 2]. An FM-QCP then exists when it is possible to shift the Curie transition temperature T C of a ferromagnet continuously to zero where a second order quantum phase transition takes place. Quantum phase transitions occur at zero entropy and are driven by quantum rather than thermal fluctuations. These fluctuations diverge at the QCP modifying the excitation spectrum of a metal and leading to a fundamental instability of Landau's Fermi liquid (FL) [3]. Typical signatures of such a behavior are observed in magnetic, thermal and transport properties and are referred to as non-Fermi-liquid (NFL) phenomena [4].Although there is clear evidence for the existence of antiferromagnetic (AFM) QCPs, the FM-QCP case is controversial. In recent years, substantial experimental and theoretical efforts were made to further investigate this problem. However, a wide range of possibilities exists. On theoretical grounds, a 3-dimensional (3D) FM-QCP is believed to be inherently unstable, either towards a first order phase transition or towards an inhomogeneous magnetic phase (modulated/textured structures) [5][6][7]. Similar results have been obtained in 2D [5,8,9] CeRuPO [21] where the FM transition temperature is suppressed to T = 0 by hydrostatic pressure, exhibit a change into AFM order before reaching the QCP. There are Ce-based alloys (CePd 1−x Rh x [22]) and also d-electron metals (Ni 1−x V x [23]) where it seems that local disorder-driven mechanisms such as Kondo disorder or the quantum Griffiths phase (QGP) scenario are responsible for the NFL properties [24][25][26]. Broad and strongly T dependent NMR and µSR linewidths are indicative for such disorder-driven mechanisms. As a consequence spin-glass-like behavior is often found, e.g., in CePd 1−x Rh x , and power-law corrections to the thermodynamic and transport properties as well as in the local spi...

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We present a detailed investigation of the magnetic and superconducting properties of Ca 1−x Na x Fe 2 As 2 single crystals with x = 0.00, 0.35, 0.50, and 0.67 by means of the local probe techniques Mössbauer spectroscopy and muon spin relaxation experiments. With increasing Na-substitution level, the magnetic order parameter is suppressed. For x = 0.50 we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature T c. A systematic comparison with other 122 pnictides reveals a square-root correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures T c /T N , which can be understood in the framework of a Landau theory. In the optimally doped sample with T c ≈ 34 K, diluted magnetism is found and the temperature dependence of the penetration depth and superfluid density are obtained, proving the presence of two superconducting s-wave gaps.